He was presented as a relentless beast that roared like a dinosaur and struck terror into the hearts of all the chipmunks, even moreso than their other main enemy, the hawk.
The fox first appears when Nono and Glikko make it to the snowy mountains and get attacked by the hawk. The fox intervenes and battles the hawk. While the two predators are fighting, the chipmunks escape. After they're gone, the fox kills the hawk, and with blood-drenched lips turns and looks ominously in the direction Nono and Glikko went.
At the end of the film, when the pair are reunited with the other chipmunks in the forest, someone sees the fox coming, having followed Nono and Glikko. The chipmunks flee in terror from the beast, whilst Glikko, blaming himself for leading the fox to where all the chipmunks live, tries to fend him off. Nono attempts to help, but is vicious batted aside. So too does Pipo the pigeon, but the fox grievously wounds him. Both of them survive and the fox pursues Glikko to the edge of a cliff.
Jutting from the cliff there was a branch and Glikko scurried out onto it. The fox followed, threatening to snap the branch with his weight. Across the gulf was Glikko's only salvation; the other side of the ravine. The other chipmunks, looking on, called for their friend to jump. Glikko finally leaped - just as the fox did, too, snapping the branch.
The fox's claws raked across Glikko's back in midair, but only succeeded in tearing Glikko's bandana. Glikko made it safely across while the chipmunks' most dreaded enemy fell victim to gravity; the fox and the broken branch plummetted soundlessly down the gorge and out of sight.